Cyanobacteria Boring Limestones in Freshwater Settings—Their Pioneering Role in Sculpturing Pebbles and Carbonate Dissolution

In freshwater lakes and rivers, cyanobacteria belonging to the family Leptolyngbyaceae bore > 1 mm deep into limestone peb-bles by dissolving carbonate at the tip of their 3–8 μm-thick filaments. The abundance of these borings decreases downwardwhile it is so high at the rock surface that micrometric debris is formed. Moreover, the disintegrated material on the pebbles'surface can be easily removed, for instance, when pebbles are grinding against each other due to wave or current action orwhen insect larvae settle and scratch loosened grains from the surface while constructing their cases. After a larvae case hasbeen abandoned, it decays with time and the surface benath it is colonized again by boring cyanobacteria. These processescan alternate repeatedly and lead to a sculptured appearance of the pebbles, especially because insect larvae tend to colonizealready existing depressions where they are better protected from predation and where they can access suspended food moreeasily. In the sculptures entrenched by insect larvae, larvae of byssate bivalves like Dreissena polymorpha may settle. Whengrowing, these bivalves also remove loosened carbonate from the bored surface. Thus, boring cyanobacteria play a pioneering,preconditioning role in the morphological evolution of limestone (pebble) surfaces by transforming an initially hard substrateinto a firm- to softground that is subsequently colonized and structured by animals. Consequently, sculptured pebbles are theproduct of multiphase, preconditioned bioerosion. Ultimately, the synergistic effects of these bioerosive processes result in thedissolution of carbonate leading to a maximum take-up of approximately 0.5–0.8 kg CO 2 per square meter and year, as a prelim-inary estimate indicates.